Breeding animals can increase the survival of their offspring by defending their offspring and perform a conspicuous display against nest predators and potential risks. Here, we recorded the behavioral responses of Antarctic penguins when a stuffed skua and a human approached their nests. We investigated (1) how sympatrically breeding chinstrap (Pygoscelis antarcticus) and gentoo (Pygoscelis papua) penguins responded to the approaching skua (a real nest predator) and human (a newly introduced intruder) and (2) how the penguin responses varied based on the degree of previous exposure to humans. Our results showed that chinstrap penguins mostly displayed strongly threatening behavior, with physical attacks on both the skua and human. However, gentoo penguins displayed a weaker threatening behavior toward the skua and responded differently to the presence of a human. Many gentoo individuals avoided the human rather than displaying threatening behavior. Furthermore, gentoo penguins near the pathway used by humans exhibited weak responses after 4 weeks of exposures to passers-by, whereas other individuals far from the pathway responded with threatening behavior. These results indicate that chinstrap and gentoo penguins may have different strategies for defending their offspring;gentoo penguins might discriminate between intruder types depending on the degree of danger, whereas chinstrap penguins consistently reacted to any intruders approaching to their nest sites. Our findings suggest that gentoo penguins may become habituated with humans following prior to shortterm exposure.